1. Field of the Invention
The present invention relates to the formation of investment casting molds by the lost wax process and, more particularly, to a method and apparatus for drying layers of ceramic slurry on a pattern of the article to be cast.
2. Description of the Prior Art
The lost wax process for forming investment casting molds is well known in the prior art and involves dipping an expendable pattern of the article to be cast into a slurry of ceramic particles, drying the layer of slurry on the pattern and repeating the sequence until the desired thickness for a mold wall is obtained. Oftentimes, dry particulate ceramic material is applied to the wet layer of slurry before it is dried to effect more rapid buildup of the wall. After the desired wall thickness is obtained, the pattern is removed and the ceramic layers are heated for consolidation into a strong mold to be used in casting.
Drying of the layers of ceramic slurry is one of the most critical steps in the process and is one of the most troublesome. Mold defects, such as cracking, flaking, bulging and the like, are frequently encountered and result in high mold rejection rates. The most common cause of such defects is the premature drying and consequent harmful overheating and expansion of those portions of the pattern which are easiest to dry. For example, in drying a layer of ceramic slurry on a wax pattern of a gas turbine blade or vane, it has been observed that the airfoil portion of the pattern dries much faster than the root or shroud portions and that the airfoil portion is more prone to overheating. Further, if the part is to be cast by directional solidification techniques, such as described in U.S. Pat. No. 3,260,505, wherein the mold is provided with an integral base, it has been observed that the base is one of the most difficult to dry areas of the assembly as a result of gravitational migration of moisture from the upper pattern surfaces to the base. In this case, the layer of slurry on the pattern may be adequately dried long before that on the base.
Attempts by prior art workers to limit the frequency of mold defects which originate during the drying step are exemplified by U.S. Pat. Nos. 2,932,864; 3,191,250 and 3,850,224. The drying process and apparatus of the last-cited patent appear to have been the most successful and involve conveying patterns coated with a layer of ceramic slurry through a U-shaped tunnel having two leg sections connected at one end by an impact drying section and open at the other end to a work room. High velocity drying air is directed laterally over the patterns in the impact drying section and then travels down each tunnel leg to effect further drying of the patterns therein. Drying is achieved by controlling the temperature and humidity of the air entering the impact drying section such that the wet bulb temperature is equal to the initial pattern temperature and is at least 10.degree. F. below the dry bulb temperature. Each layer of ceramic slurry is dried in a separate tunnel, the wet bulb temperature of the drying air being held substantially constant from tunnel to tunnel while the dry bulb temperature is progressively increased. Although the process and apparatus of U.S. Pat. No. 3,850,224 and the other cited patents are improvements over the prior art, they nevertheless suffer from numerous disadvantages.
First, the drying air circulating through the tunnel is conditioned and controlled only at the entrance to the impact drying section. There is no provision for varying the temperature, humidity or velocity of the drying air after it enters the system in response to changes in the drying kinetics of the slurry layer. Also, there is no provision for ensuring that the humidity of the drying air in each section of the tunnel is uniform. As the coated patterns in the leg and impact drying sections dry and release moisture, it is possible to have drying air of different humidity in different sections of the tunnel. This lack of uniformity makes precise control over the drying process extremely difficult to achieve. Second, large patterns or clusters of multiple patterns tend to shield one another from the longitudinal airflow in the tunnel legs. This shielding inhibits even and complete drying of the patterns. Third, the exact drying time which is best for each layer of ceramic slurry cannot be achieved because all the tunnels are of the same length and the conveyor speed at each tunnel is the same. Fourth, there is no provision for adjusting the drying parameters to particular pattern shapes and sizes. Large patterns requiring long drying times and small patterns requiring much less drying time are subjected to similar drying schedules. In addition, all patterns, regardless of size and shape, are subjected to the same airflow distribution in the tunnel. No provision is made for adjusting the direction of airflow to concentrate airflow differently on different pattern shapes. These, as well as other, disadvantages severely limit the effectiveness of the prior art systems in reducing the incidence of mold defects originating during the drying step of the mold formation process.